A) Field of the Invention
This invention relates to a micro lens array suited for restraining dispersion of an optical beam such a laser beam and a manufacturing method of the micro lens array.
B) Description of the Related Art
Conventionally a scanning printing (or recording) device such as a laser printer, etc. uses a semiconductor laser shown in FIG. 23 as a light source.
The semiconductor laser emits a laser beam 3 from a laser active layer 2 on a side surface 1A of a semiconductor substrate 1. The active layer 2 is configured to have a belt shape on the substrate side surface 1A. Defining a longitudinal direction L and a direction of thickness t of the active layer 2 respectively as an x-axis and a y-axis, an emitting direction of the laser beam 3 will be a positive direction on a z-axis crossing with an x-y coordinate surface (orthogonal coordinates surface) at a right angle. The x-axis, y-axis and z-axis are respectively corresponding to a slow axis, a fast axis and an optical axis in a laser scanning.
The laser beam 3 normally disperses by about 10 degrees in the x-axis direction and by about 20-30 degrees in the y-axis direction. In order to restrain these dispersion of the laser beam and to concentrate light onto an edge of an optical fiber, etc., for example, it is well known to use an optical system as shown in FIG. 24 (refer to Japanese Laid-Open Patent H09-96760).
In the optical system shown in FIG. 24, laser beams 3a-3c emitted from the semiconductor laser 1 are concentrated into the y-axis direction by a semi-cylindrical lens surface 5 formed on one principal surface of a lens substrate 4 and concentrated into the x-axis direction respectively by spherical lens surfaces 6a-6c formed on another principal surface of the lens substrate 4. The concentrated laser beams 3a-3c propagate to the z-axis direction and irradiated to edge surfaces of optical fibers 7a-7c. It is possible to form the lens surface 5 (or the lens surfaces 6a-6c) on a separate lens substrate from the lens substrate 4 having the lens surfaces 6a-6c (or the lens surface 5). However, if the lens surface 5 and the lens surfaces 6a-6c are unitedly formed on the lens substrate 4 as shown in FIG. 24 and at the same time a radius of curvatures of the lens surface 5 and the lens surfaces 6a-6c independently form each other in accordance with a divergence angle of the laser beam, it will be possible to realize a small and high-performance micro lens array for concentrating light.
As for an optical system for restraining light dispersion, the optical system shown in FIG. 25 has been well known (refer to published Japanese translation of a PCT application JPA 2002-513959). Laser beams emitted from active layers 3A, 3B and 3C of a semiconductor laser 1 irradiate to a lens substrate 9 via a lens substrate 8 having a cylindrical lens 8S. A cylinder axis of the lens 8S extends to the x-axis direction. Cylindrical lens surfaces 9A, 9B and 9C are formed respectively in correspondence with the active layers 3A, 3B and 3C on one principal surface of the lens substrate 9 opposing to the lens 8S, and cylindrical lens surfaces 9a, 9b and 9c are formed respectively in correspondence with the lens surfaces 9A, 9B and 9C on another principal surface of the lens substrate 9. The cylinder axes of the lens surfaces 9A, 9B and 9C and the lens surfaces 9a, 9b and 9c extend to the y-axis direction.
FIG. 26 shows collimation in the y-axis direction of the optical system shown in FIG. 25. The reference number “3y” in the drawing represents an active part corresponding to a thickness t of one active layer (e.g., the active layer 3A). A laser beam emitted from the active part 3y is collimated by the lens 8S and transmitted via the lens substrate 9.
FIG. 27 shows formation of beam waist in the x-axis direction of the optical system shown in FIG. 25. An active layer 3x corresponds to the length L of one active layer (e.g., the active layer 3A). A laser beam emitted from the active layer 3x is irradiated to the lens substrate 9 via the lens 8S. In the lens substrate 9, the laser beam is refracted by the lens 9A and thereafter the beam waist BW is formed by the telescope effect near the center of the substrate and the laser beam again is refracted by the lens 9a. Where a dispersion angle on an irradiating side of the lens 9A is “α” and a beam width and a dispersion angle on an emitting side of the lens 9a are respectively “Lo” and “β”, “αL” will be “βLo” (αL=βLo) by Lagrange Invariant, and “β” will be smaller than “α” (β<α), i.e., the divergence angle “β” will be smaller than the divergence angel “α”, because “Lo” is larger than “L” (Lo>L).
Conventionally it is well known that a laser beam shaper converts a laser beam having an oval cross section into a laser beam having a circle cross section by passing the laser beam through one side surface to another side surface of a cylindrical transparent body (refer to published Japanese translation of a PCT application JPA H09-501789). In this case, a cylindrical concave lens surface is formed on one side surface (irradiated surface) of the cylindrical transparent body, and a toroidal lens surface (convex lens surface in a donut-like shape wherein radiuses of curvature are different in two different directions crossing with each other at a right angle) is formed on another side surface (emitting surface) of the cylindrical transparent body.
According to the above-described conventional technique shown in FIG. 24, when the lens substrate 4 having the lens surfaces 5 and 6a-6c is used as a collimator, it is necessary to arrange the semiconductor laser 1 sufficiently apart from the lens substrate 4. By arranging the semiconductor substrate 1 apart from the lens substrate 4, adjoining laser beams such as the laser beams 3a and 3b may be overlapped with each other, and a necessary arrangement space may be increased. The lens substrate 4 having the lens surfaces 5 and 6a-6c dose not have the telescope effect as in FIG. 27; therefore, the beam waist cannot be formed inside the substrate 4.
According to the above-described conventional technique shown in FIG. 25, it is necessary to define positions of two lens substrates 8 and 9 precisely toward the semiconductor laser 1. Therefore, it takes more time to position two substrates and a positioning gap may be easily occurred by change in an environment. Moreover, it needs more parts so that a manufacturing cost will be increased, and miniaturization of the optical system will be restrained.
According to the above-described laser beam shaper, addition of a collimator lens might be necessary to further decrease an opening angle of the laser beam emitted from an emitting surface. Furthermore, the above-described laser beam shaper does not have the telescope effect as in FIG. 27.